This is the third book in the Mercy Thompson series by Patricia Briggs.
The main plot issue in this one involves the fae; the vamps aren't really in evidence at all except as something Mercy worries about because of events in the second book. There's been a series of murders on one of the fae reservations and they think that Mercy will be able to see through whatever magic is protecting the murderer and help them find whomever it is.
There's a lot in this one about the policing of people who can't really be bound by "human" laws. How do you punish someone who can't really be held captive and is essentially immortal? Do the humans let them take care of their own in whatever way they think is necessary? Or do they insist on following the law of the land? If your leaders tell you to sacrifice yourself for the good of your people... do you do it? Even if you're innocent?
There's also some incredibly tough stuff near the end between Adam, Mercy, and Ben. It explains a lot about why Ben is the way he is and that there is a good person inside he's just completely buried it. But oh man did I cry through that part! Both for Ben and for what Mercy went through.